The invention relates in general to the technological field of packet-switched cellular radio systems. More precisely the invention relates to the cell change procedures, i.e. the procedures for rerouting an active packet-switched communication connection between a mobile station and a fixed packet-switched network through a new base station. An important application framework for the invention is the GPRS or General Packet Radio Service system which is undergoing specification at the priority date of the present patent application. Other at least equally important application frameworks are the UMTS (Universal Mobile Telecommunication System) where the so-called connectionless data services are packet-switched, and the EDGE (Enhanced Data rates for GSM evolution) where also connectionless data services will be implemented.
The services that are offered over telecommunication connections may be broadly categorized into real time and non-real time services. The former class covers such services where an essentially continuous or at least piecewise continuous stream of information is carried from the transmitter to the receiver and the intelligibility of the received information depends on the continuity: only relatively small and relatively constant delays are allowed. Typical real time services involve the transmission of speech and (moving) images like in a telephone or videophone connection, where e.g. a voice activity detection system may be used to provide piecewise continuity, i.e. to periodically halt the transmission when there is nothing important to transmit. Non-real time services cover the transmission of information with less critical timing requirements and are typically referred to as transmission of data. Electronic mail and downloading of files from or to a distant location are examples of non-real time service applications.
Packet-switched cellular radio systems have usually been designed to complement the known circuit-switched mobile telephone systems and their future successors so that the division of work would involve the use of circuit-switched systems to provide real time services and packet-switched systems to provide non-real time services. Circuit- and packet-switched transmission may even be implemented as alternative operational modes within the framework of a single telecommunication systems, or in hybrid network arrangements where the same base stations and other radio access hardware are used for both systems but the other fixed network devices and communication connections between them are separate. An arrangement of the last mentioned type is the combination of a GSM mobile telephone network (Global System for Mobile telecommunications) and the GPRS network.
FIG. 1 illustrates a combined GSM/GPRS arrangement where an MS or mobile station 101 may choose one of the BSs or base stations 102 to 107 to communicate with. Base stations 102 and 103 operate under a controlling device 108 where a PCU or packet control unit is combined to a BSC or base station controller. Similarly base stations 104 and 105 operate under controlling device 109 and base stations 106 and 107 operate under controlling device 110. From this level upwards the GSM arid GPRS networks have different architectures. Each base station controller is coupled to a MSC or a mobile switching centre and these are in turn interconnected through a GSM transmission network 115. Each packet control unit is coupled to a SGSN or serving GPRS support node and these are in turn interconnected through a GPRS transmission network 116, where the connections go through GGSNs or gateway GPRS support nodes (not shown). The small number of devices shown in FIG. 1 is for graphical clarity only: in realistic GSM/GPRS systems there are typically thousands of base stations, hundreds of BSC/PCUs and dozens of MSCs and SGSNs.
It should be noted that the location of a PCU is not defined very strictly in the known GPRS: in addition to the arrangement shown in FIG. 1 a PCU may be located in a base station or in an SGSN. The present invention does not have any particular biasing towards any most advantageous location of the PCU. The general network architecture and the interconnections of the base stations, PCUs and SGSNs naturally depend on the location of the SGSN.
Around each base station there is a cell or radio coverage area within which a mobile station may communicate with that base station. When a mobile station moves out of one cell and into another, a cell reselection, a handover or a cell change must be performed. Cell reselection takes place when the mobile station is in idle mode and no communication connections are currently active between it and the fixed parts of the network. A handover means that an active circuit-switched communication connection must be rerouted through a new base station. A cell change means that an active packet-switched communication connection must be rerouted through a new base station. Regarding packet-switched (GPRS) communications in an arrangement according to FIG. 1 the cell reselection or cell change may be intra-PCU (for example from base station 102 to base station 103), inter-PCU but intra-SGSN (for example from base station 103 to base station 104) or inter-PCU and inter-SGSN (for example from base station 105 to base station
Although the GPRS system like many other packet-switched cellular radio systems has been designed primarily for providing non-real time services, there is a tendency towards using it also to provide some real time services, of which the Internet calls are a good example. An Internet call is a telephone connection routed through the Internet instead of the common telephone networks. It may be accompanied with a video part. Also other applications requiring real time services through a packets switched cellular radio network are known.
A problem of the known GPRS that also appears in many other packet-switched cellular radio systems is that the cell change procedures have been opimized for simplicity rather than short delay. According to the standardized GPRS procedure the exchange of packet data between a mobile station and the old base station is first terminated, after that a new cell is chosen and only thereafter the transfer of packets is resumed through the base station of the new cell. The procedure works well for non-real time services since the delay between terminating the old connection and establishing the new one does not matter. However, the length of the delay is easily several hundreds of milliseconds or even up to some seconds, which is definitely too much for a real time service where such a long delay is at least annoying and may even cause the call or other connection to be dropped if the delay is interpreted as a severe breakdown in the flow of information. The long delay may also cause frustration to the users of non-real time services even when it does not actually affect the intelligibility of the transmitted information: for example the downloading of web pages is unnecessarily slowed down by the delays.
It is an object of the present invention to provide a method and an arrangement for performing a cell change in a packet-switched cellular radio network with a delay that is short enough to be acceptable for real time services and to avoid unnecessary retardation for non-real time services.
The objects of the invention are achieved by commencing the cell change before terminating the old connection and preferably also using a countdown timer to trigger the switchover from the old cell to the new cell.
The method of the invention is designed for implementing a cell change for a mobile station in a packet-switched cellular radio system comprising a first base station, a second base station and a controlling unit controlling the operation of at least the first base station. It comprises as its characteristic features the steps of
establishing at the controlling unit the knowledge about the mobile station""s need for performing a cell change from the cell of the first base station to the cell of the second base station while the mobile station is still communicating with the first base station,
transmitting from the controlling unit towards the mobile station through the first base station a first message in order to fix an oncoming first moment of time as the moment of performing cell change and
from said first moment of time onwards providing access for the mobile station to the cell of the second base station.
Additionally the invention applies to a method for implementing a cell change in a mobile station of a packet-switched cellular radio system This aspect of the invention comprises as its characteristic features the steps of
receiving a message from a controlling unit of the packet-switched cellular radio system through a first base station,
after the reception of said message, continuing the utilization of an existing packet-switched communication connection with the first base station until a certain first moment of time defined in said message as that moment of performing cell change and
from said first moment of time onwards accessing the cell of a second base station.
The invention also applies to an arrangement for implementing a cell change for a mobile station in a packet-switched cellular radio system comprising a first base station, a second base station and a controlling unit for controlling the operation of at least the first base station. The arrangement comprises as its characteristic features
in the controlling unit means for establishing at the controlling unit the knowledge about the mobile station""s need for performing a cell change from the cell of the first base station to the cell of the second base station while the mobile station is still communicating with the first base station,
in the controlling unit and the first base station means for transmitting towards the mobile station through the first base station a first message in order to fix an oncoming first moment of time as the moment of performing cell change and
in the second base station means for from said first moment of time onwards providing access for the mobile station to the cell of the second base station.
The fact that a cell change may be initiated either by the mobile station or by the network is known as such. According to the invention the initiation of cell change does not have the immediate effect of terminating the packet-switched transmission of data between the mobile station and the old base station. Instead, the allocation of resources from the new base station is commenced while the old connection is still active. A cell change command message is transmitted to the mobile station. This message causes the mobile station to start a countdown timer the expiry of which corresponds to the exact estimated moment for changing cells.
The cell change command message may also comprise a reference or temporary identifier switch the mobile station may use when it accesses the new base station. This is a particularly simple way of ensuring that the mobile station to which the newly allocated resources are given from the new base station actually is the one for which the cell change was requested. Also the obligatorily defined moment of time when the mobile station must access the new base station may be communicated to both the mobile station and the new base station.